The present invention is directed to capacitor discharge engine ignition systems for small two and four stroke engines used in chain saw and weed trimmer applications, for example. The invention is more specifically directed to automatic control of engine ignition timing to obtain spark advance between starting and normal operating speeds, and to retard timing and thereby limit operation at excess engine operating speed.
The time and occurrence of engine ignition is of importance to startability, output power and emissions performance of engines, including small two and four stroke engines. Optimum engine ignition timing varies, primarily as a function of engine speed and engine load. Secondary factors, such as emissions performance and fuel quality, also play a role in determining optimum spark timing. Mechanical and microprocessor-based electronic timing control systems have been proposed for large engine applications, such as automotive engines, but are not well suited to small engine applications because of cost and packaging factors. Specifically, it has been proposed to employ microprocessor-based ignition modules in small engine applications, in which desired advance and/or retard timing characteristics are programmed into the microprocessor. However, cost factors associated with microprocessor-based modules are prohibitive in most small engine applications.
It has also been recognized that there is a danger to the integrity of the engine at excess operating speed. It is possible for the engine, particularly when there is either no load or a load that has been suddenly removed, to accelerate to an rpm range at which the engine components can be damaged. Carburetor ball-type speed governors are conventionally employed, having a spring-loaded ball that is sensitive to engine vibration. The level of vibration is, in turn, sensitive to engine speed. When vibration-induced forces on the ball overcome spring pressure, fuel is added to the engine. This sudden enrichment of the air/fuel ratio slows the engine, but produces increased emissions from the engine exhaust. Electronic systems have been proposed for disabling ignition in the event of excess engine speed, as disclosed for example in U.S. Pat. No. 5,245,965. However, every missed spark represents a charge of air and fuel that is not burned in the engine. This unburned fuel exits the engine and enters the exhaust system. The unburned fuel and air leave the exhaust system as unburned hydrocarbon emissions, causing an increase in air pollution. The spark suppression technique also causes mis-operation of the engine, increasing engine vibration and potentially suggesting malfunction of the engine to a user. Both the ball speed governor and the electronic skip spark governor result in unburned fuel and air entering the exhaust system. In catalytic converter- equipped engines, this fuel is oxidized catalytically in the converter, which increases the temperature of the converter. Converter technology in small engine applications is limited in size and allowable percentage of effectiveness, so that any fuel oxidation can greatly reduce effectiveness of the catalytic process.
It is an object of the present invention to provide a capacitor discharge ignition system that is particularly well suited for small engine applications, which eliminates kick-back during starting, which facilitates manual starting of the engine, which includes facility for automatically preventing over-speed operation of the engine while reducing delivery of unburned fuel to the exhaust system, which is relatively inexpensive, and/or which is well adapted for use in small two stroke and four stroke engine applications.
A capacitor discharge engine ignition system in accordance with a presently preferred embodiment of the invention includes an ignition coil having a primary winding and a secondary winding for coupling to an engine ignition spark plug. A first electronic switch has primary current conducting electrodes in circuit with an ignition charge storage capacitor and the primary winding of the ignition coil, and a control electrode responsive to trigger signals for operatively connecting the ignition charge storage capacitor to discharge through the primary winding of the ignition coil. A charge/trigger coil arrangement generates periodic signals in synchronism with operation of the engine. The charge coil generates a charge signal to charge the ignition charge storage capacitor, while the trigger coil generates a trigger signal for triggering discharge of the capacitor through the ignition coil. An electronic circuit for controlling timing of the trigger signal as a function of engine speed includes a second electronic switch having primary current conducting electrodes operatively connected to the control electrode of the first electronic switch, and a control electrode. An RC circuit, including a resistor and a capacitor, is operatively connected to the charge coil and the control electrode of the second electronic switch to prevent application of the trigger signal to the control electrode of the first electronic switch during occurrence of the charge signal, and thereby controlling timing of application of the trigger signal to the control electrode of the first electronic switch as a function of engine speed.
The electronic circuit for controlling timing of the trigger signal as a function of engine speed in the preferred embodiment of the invention obtains both automatic spark advance between engine starting and normal operating speed, and engine ignition retard at excess engine operating speed. The charge coil and the trigger coil are constructed and arranged such that a trigger signal is generated in the trigger coil both before and after each charge signal is generated in the charge coil, but the charge on the capacitor of the RC engine timing circuit prevents application of the second trigger signal to the control electrode of the first switch, so that the charge on the ignition""s charge storage capacitor is held until occurrence of the next trigger signal series. Timing of the leading trigger signal in the next series automatically advances as a function of increasing engine speed, so as to obtain an automatic spark advance with increasing engine speed between starting and normal operating speed. This automatic advance varies approximately linearly to a maximum advance in the range of 20xc2x0 to 40xc2x0. In the event of excess engine speed, the charge on the capacitor of the RC ignition timing circuit does not have an opportunity fully to discharge, so that engine ignition is automatically retarded. However, ignition is not prevented, so unburned fuel is not fed to the engine exhaust system. Furthermore, engine ignition is prevented in the event of reverse engine operation.